The present invention relates to the art of electromagnetic field correction and modification. It finds particular application in conjunction with the maintenance of gradient magnetic field spatial linearity in magnetic resonance imaging and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with magnetic resonance spectroscopy and other applications in which electromagnetic field linearity is adjusted or controlled.
In magnetic resonance imaging and spectroscopy, a uniform magnetic field is created through an examination region in which a subject to be examined is disposed. The magnetic field through the examination region preferentially aligns the magnetization of the dipoles of the subject with the uniform field. Radio frequency excitation pulses are applied to cause the magnetization to precess about the uniform magnetic field. After the radio frequency excitation, radio frequency magnetic resonance signals are generated as the precessing magnetization decays back toward alignment with the uniform magnetic field. The frequency of the radio frequency magnetic resonance signals is proportional to the strength of the magnetic field and the gyromagnetic ratio of the precessing dipole. Various combination of radio frequency pulses and magnetic field gradient pulses are applied to manipulate the precessing magnetization to create echo signals and the like.
In magnetic resonance imaging, gradient magnetic fields are applied to select and encode subregions in the subject. Some magnetic field gradients are applied to select one or more slices or planes to be imaged. Other magnetic field gradients selectively modify the uniform field to encode positionally related information into the frequency and phase of the magnetic resonance signals.
The gradient fields are conventionally applied as a series of gradient pulses. The duration and timing of the pulses relative to the radio frequency signals is precisely timed to optimize performance. More specifically, electric current pulses of the selected duration, amplitude, shape, and timing are applied to gradient field coils to cause corresponding gradient magnetic field pulses.
One of the problems is that the resultant gradient magnetic field pulses do not match the profile of the electric current pulses applied to the gradient field coils. A changing magnetic field inherently induces eddy currents, which eddy currents cause corresponding eddy magnetic fields. Thus, each applied current pulse causes a gradient magnetic field pulse which causes eddy currents that add unwanted eddy components to the induced gradient magnetic field pulse. The eddy current effect varies with the amount and conductivity of the material in which the eddy currents are induced, the proximity of this material to the gradient field coil, and the magnitude of the pulsed gradient magnetic field. The conductive structures might include supporting structures for the magnets, a liquid nitrogen dewar of a superconducting magnet, control panels, room structures, and other adjacent metallic or conductive structures. The variances in the thickness, concentricity, and other construction tolerances, from magnet to magnet, cause each magnetic resonance system to generate spatially variant eddy currents with different magnitudes and time constants.
In order to improve the imaging quality, the shape of the electric current pulse is altered such that the magnetic field produced by the sum of the altered current pulse and the eddy currents more closely matches the desired gradient magnetic field pulse shape and duration. One such correction circuit is illustrated in U.S. Pat. No. 4,703,275, issued Oct. 27, 1987 to G. Neil Holland. In this patent, the current pulse is divided among a plurality of parallel connected correction paths. Each correction path includes a band pass filter which has an adjustable centre frequency and a variable gain amplifier. More specifically, the band pass filter includes an RC circuit whose time constant is adjustable to adjust the effective centre frequency of the filter. By appropriately adjusting the RC time constant or frequency of the band pass filter and the gain of the amplifier, appropriate compensation can be made for a corresponding eddy current. The output of the parallel correction paths are summed to produce the modified gradient current pulse.
To calibrate the frequency and amplitude adjustment of each of the current adjustment paths heretofore, a search coil was disposed in the magnetic field and connected by an integrater to one channel of a dual channel oscilloscope. The summed output of the correction channels was connected to the second channel of the oscilloscope. The RC time constant of frequency and the gain of each correction path were adjusted until the gradient of the selected wave form was produced.
This calibration method was effective for high and middle frequency eddy currents, i.e. those with relatively short decay constants. However, this method tends to be difficult and less accurate for low frequency eddy currents, i.e. those with long decay times. Although this method works well for decay times in the millisecond range, it becomes significantly less effective as the decay times approach the tenths of a second and seconds range.
The present invention provides a new and improved technique for calibrating eddy current compensation circuitry that is particularly effective for low frequency eddy currents.